People have long speculated
about the possibility of life in settings other than Earth. Only in the
past few centuries, however, have we been able to conceive of the
specific nature of such settings: other planets around our own sun and
solar systems similar to our own elsewhere in the physical universe.
Speculation on the nature of life elsewhere often has paid little heed
to constraints imposed by the nature of biochemistry, however. A
century of fanciful science fiction has resulted not only in social
enthusiasm for the quest for extraterrestrial life, but also in
fanciful notions of the chemical and physical forms that life can take,
what the nature of life can be. Since the time of the Viking missions
to Mars, in the mid-1970s, our view of life's diversity on Earth has
expanded significantly, and we have a better understanding of the
extreme conditions that limit life. Consequently, our search for extant
life elsewhere in the solar system can now be conducted with broader
perspective than before.

How can life be detected regardless of its nature and origin?
Considering the recent spectacular advances in observational astronomy,
it seems likely that the first sign of life elsewhere will be the
spectroscopic detection of co-occurring nonequilibrium gases, for
instance oxygen and methane, in the atmosphere of a planet around some
distant star. Co-occurrence of such gases would indicate that they are
replenished, perhaps most readily explained by the influence of life
(1). By observation of oxygen and methane, Earth could possibly be seen
as a home for life even from distant galaxies. Other potential habitats
for life in this solar system, such as Mars and Europa, however, are
not so obvious. The search for life on those bodies will be conducted
at the level of analytical chemistry. As we undertake the detection of
extraterrestrial life, it is instructive to try to put constraints on
what the nature of life can be. These constraints, the requirements for
life, tell us where and how to look for life, and the forms that it can
take.

What Is Life?

An early question that needs to be confronted, indeed a
question that in the last analysis requires definition, is: What is
life? Most biologists would agree that self-replication, genetic
continuity, is a fundamental trait of the life process. Systems that
generally would be deemed nonbiological can exhibit a sort of
self-replication, however (2). Examples would be the growth of a
crystal lattice or a propagating clay structure. Crystals and clays
propagate, unquestionably, but life they are not. There is no locus of
genetic continuity, no organism. Such systems do not evolve, do not
change in genetic ways to meet new challenges. Consequently, the
definition of life should include the capacity for evolution as well as
self-replication. Indeed, the mechanism of evolution—natural
selection—is a consequence of the necessarily competing drives for
self-replication that are manifest in all organisms. The definition
based on those processes, then, would be that life is any
self-replicating, evolving system.

The processes of self-replication and evolution are not reliably
detectable, even in the terrestrial setting. Consequently, in the
practical search for life elsewhere we need to incorporate information
on the nature of the chemistries that can provide the basis for
self-replication and evolution. Considering the properties of molecules
likely to be needed to replicate and evolve, it is predictable that
life that we encounter anywhere in the universe will be composed of
organic chemicals that follow the same general principles as our own
organic-based terrestrial life. The operational definition of life then
becomes: Life is a self-replicating, evolving system expected to be
based on organic chemistry.

Why Organic Chemistry?

The basic drive of life is to make more of itself. The
chemical reactions required for the faithful propagation of a
free-living organism necessarily require high degrees of specificity in
the interactions of the molecules that carry out the propagation. Such
specificity requires information, in the form of complex molecular
structure—large molecules. The molecules that serve terrestrial
organisms typically are very large, proteins and RNAs with molecular
weights of thousands to millions of daltons, or even larger as in the
case of genetic DNA. It is predictable that life, wherever we encounter
it, will be composed of macromolecules.

Only two of the natural atoms, carbon and silicon, are known to
serve as the backbones of molecules sufficiently large to carry
biological information. Thought on the chemistry of life generally has
focused on carbon as unique (3). As the structural basis for life, one
of carbon's important features is that unlike silicon it can readily
engage in the formation of chemical bonds with many other atoms,
thereby allowing for the chemical versatility required to conduct the
reactions of biological metabolism and propagation. The various organic
functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus,
sulfur, and a host of metals, such as iron, magnesium, and zinc,
provide the enormous diversity of chemical reactions necessarily
catalyzed by a living organism. Silicon, in contrast, interacts with
only a few other atoms, and the large silicon molecules are monotonous
compared with the combinatorial universe of organic macromolecules.

Life also must capture energy and transform that energy into the
chemistry of replication. The electronic properties of carbon, unlike
silicon, readily allow the formation of double or even triple bonds
with other atoms. These chemical bonds allow the capture and
delocalization of electronic energy. Some carbon-containing compounds,
therefore, can be highly polarized and thereby capture “resonance
energy” and transform this chemical energy to do work or to produce
new chemicals in a catalytic manner. The potential polarizability of
organic compounds also contributes to the specificity of intermolecular
interactions, because ionic and van der Waals complementarities can
shift to mesh with or to repulse one another. Finally, it is critical
that organic reactions, in contrast to silicon-based reactions, are
broadly amenable to aqueous conditions. Several of its properties
indicate that water is likely to be the milieu for life anywhere in the
universe (2).

The likelihood that life throughout the universe is probably
carbon-based is encouraged by the fact that carbon is one of the most
abundant of the higher elements. Astronomical studies find complex
organic compounds strewn throughout interstellar space. Moreover, the
common occurrence of carbonaceous meteorites testifies to an
organic-rich origin for our own solar system. If life indeed depends on
the properties of carbon, then life is expected to occur only in
association with second- or later-generation stars. This is because
carbon is formed only in the hearts of former stars, so far as we know.

The Universal Nature of Biochemistry

Life as we know it builds simple organic molecules that are
used as building blocks for large molecules. Amino acids are used to
construct the long chains of proteins; simple sugars combine with the
purine and pyrimidine bases and phosphate to construct the nucleic
acids. It seems logical that the evolution of any organic-based life
form would similarly result in the construction of complex molecules as
repeating structures of simple subunits. Indeed, it seems likely that
the basic building blocks of life anywhere will be similar to our own,
in the generality if not in the detail. Thus, the 20 common amino acids
are the simplest carbon structures imaginable that can deliver the
functional groups used in life, with properties such as repeating
structure (the peptide unit), reactivity with water, and intrinsic
chirality. Moreover, amino acids are formed readily from simple organic
compounds and occur in extraterrestrial bodies such as meteorites, so
are likely to form in any setting that results in the development of
chemical complexity necessary for life.

Similarly, the five-carbon sugars used in nucleic acids are
likely to be repeated themes, perhaps in part because they are the
smallest sugars that can cyclize and thereby confer spatial orientation
on other molecules, for instance the purines and pyrimidines that
comprise the genetic information of terrestrial organisms. Further,
because of the unique abilities of purines and pyrimidines to interact
with one another with particular specificity, these subunits, too, or
something very similar to them, are likely to be common to life
wherever it occurs. Differences in evolutionary systems likely will lie
at the higher-order levels: the structures of the large molecules
assembled from the simple units, and the mechanisms by which they are
assembled and in which they participate.

Themes that are probably common to life everywhere extend beyond
the building blocks. Energy transformation is a critical issue. The
processes of life require the capture of adequate energy, from physical
or chemical processes, to conduct the chemical transformations
requisite for life. Based on thermodynamics there are only two such
energy-capturing processes that can support “primary
productivity,” the synthesis of biological materials from inorganic
carbon dioxide. One process, termed lithotrophy, involves the oxidation
and concomitant reduction of geochemical compounds. For instance,
methanogenic organisms gain energy for growth by the use of hydrogen
(H2) as a source of high-energy electrons, which
are transferred to carbon dioxide (CO2), forming
methane (CH4). Other microbes might use hydrogen
sulfide (H2S) as an energy source, respiring with
oxygen (O2), to produce sulfuric acid
(H2SO4). It is thought that
the earliest life on Earth relied on lithotrophic metabolism.

The second general process for obtaining energy, photosynthesis,
captures light energy and converts it into energetic electrons that can
be used to accomplish biochemical tasks. Photosynthesis arose early in
the history of terrestrial life and probably drives most primary
productivity on Earth today. The contribution of lithotrophy to
terrestrial primary productivity remains unknown, however, because
there currently is little information on such organisms that may be
distributed throughout the Earth's crust, wherever the physical
conditions permit.

Although terrestrial life and life that might arise independently
of Earth are expected to use many similar, if not identical, building
blocks, they also are expected to have some biochemical qualities that
are unique. This expectation is based on the fact that different
evolutionary lines of terrestrial evolution also have engendered
novelties unique to those lines. Thus, the biochemistry of
methanogenesis arose uniquely in Archaea, whereas the property of
chlorophyll-based photosynthesis was invented among the phylogenetic
domain Bacteria (below). The cytoskeleton, which is probably a
requirement for large and complex cell structure arose in the
eukaryotes. Considering the variety of Earth's life, novelty, as
well as commonality, must be expected elsewhere.

The Physical Limits of Life: The Habitable Zone

Thought on where in the solar system life might occur was
limited historically by the belief that life relies ultimately on light
and warmth from the sun and, therefore, is restricted to the surfaces
of planets. The inner boundary of the “habitable zone” in our
solar system was considered as approximately between Earth and Venus,
not so close to the sun as to be too hot for life. The outer boundary
was considered to lie between Mars and Jupiter, not so far from the sun
that the surface of a body would necessarily be frozen or receive too
little light for efficient photosynthesis. Light probably is not
directly required for life to arise, however, except as it may be
involved in the formation of organic compounds during the accretion of
a planetary system. On the other hand, the biological use of light
energy, photosynthesis, may be a prerequisite for persistence of
planetary life over billions of years. The reason for this conjecture
is that light provides a continuous and relatively inexhaustible source
of energy. Life that depends only on chemical energy inevitably will
fail as resources diminish and cannot be renewed.

Nonetheless, we know that life occurs in Earth's crust, away
from the direct influence of light, and that many organisms have
metabolisms that function independently of light. Thus, the outer
boundary of the potentially habitable zone extends into the far reaches
of the solar system, to any rocky body with internal heating,
regardless of its distance from the sun. [I specify “rocky” body
to accommodate chemicals expected to be required for metabolism
(below).] Life can persist in the absence of light by using inorganic
energy sources, as do lithotrophic organisms, or organic sources
deposited in planetary interiors during their accretion, as do
heterotrophs (4). Therefore, rather than proximity to the sun, it seems
more useful to define the habitable zone for life in terms of the
chemical and physical conditions that are expected to be required for
life. Our view of life's possible extremes currently is limited to the
extremes of terrestrial life. Considering the intrinsic fragility of
complex organic systems, coupled with the powerful force of natural
selection, I venture that the physical limits of life are likely to be
about the same anywhere in the universe. The window of chemical and
physical settings that permit life are broad, however. Some important
considerations are the following.

Chemical Setting.

Although the general energy requirement of life is a state of
chemical disequilibrium, in which some oxidation-reduction reaction can
occur, the specific thermodynamic requirements of biological
energy-gathering strategies constrain the sites where life can occur.
For example, a setting for lithotrophic organisms requires the
occurrence of an appropriate mix of oxidized and reduced chemicals.
Photosynthetic organisms require sufficient light of appropriate
frequency. The light must be sufficiently energetic to support
biosynthesis, but not so energetic as to be chemically destructive.
These considerations constrain photosynthesis-based life to the
spectral zone of about 300–1,500 nm in wavelength. (Terrestrial
photosynthesis is limited to about 400–1,200 nm.) Beyond the
requirements for energy metabolism and CO2 as a
carbon source, terrestrial life requires only a few elements: H, N, P,
O, S, and the suite of metals.

Physical Setting.

Physical constraints on life include temperature, pressure, and volume.
The extreme diversity of terrestrial life probably provides an analog
for life's diversity anywhere.

Temperature is a critical factor for life. Temperatures must be
sufficiently high that reactions can occur, but not so high that that
complex and relatively fragile biomolecules are destroyed. Moreover,
because life probably depends universally on water, the temperature
must be in a range for water to have the properties necessary for
solute transfer. Water can be stabilized against boiling by pressure,
but at too-low temperatures, water becomes crystalline and inconsistent
with transport. Currently, the upper temperature record for culturable
microbes is 112–113°C, held by hyperthermophilic archaeons of the
genera Pyrolobus and Pyrodictium (5). Even the
spectacularly durable bacterial endospore does not survive extended
heating beyond ca. 120°C. The lower-temperature boundary
for life is not established, but microbes are recoverable from ice, and
growth of organisms has been detected in ice to −20°C (6). The
physical properties of ice can allow solute diffusion at temperatures
much lower than the freezing point (7). Thus, if based on aqueous
organic chemistry, the temperature span for life anywhere in the
universe is likely to be less than 200°C, within roughly −50° to
150°C.

Pressure and Volume.

The pressure required of a setting for life is probably limited at the
lower end only by the vapor pressure required to maintain water or ice.
An upper limit for pressure tolerance is unknown. Organisms on the
terrestrial seafloor experience pressure over 1,000 atmospheres, and
microbes recovered from deep oil wells are exposed to far higher
pressures. The upper pressure limit for life probably is determined
mainly by the effect of the pressure in reducing volume for occupancy.
Life can be remarkably small, however. It is estimated that cells only
a few hundred nanometers in diameter can contain all of the components
considered necessary for life (8).

The expected commonality of chemistry in life's processes
assists in life detection because it predicts that terrestrial types of
biochemicals are useful targets for analysis even in an
extraterrestrial setting. On the other hand, the expected similarity of
terrestrial and potentially alien life complicates the interpretation
of positive chemical tests for biochemicals. Thus, analyses of simple
terrestrial-like biochemical compounds might not distinguish between a
signal of life on one hand and an abiotically derived organic chemical,
or between an alien life form and a terrestrial contaminant.
Distinction between organisms with different evolutionary origins may
require analysis of macromolecules and genes. Particularly, the nature
and detail of the genetic information would be telling.

A Genetic Signature of Terrestrial Life

All life on Earth is genetically related through an
evolutionary past that extends beyond 3.8 billion years ago. We see
this relatedness in the many common structural and mechanistic features
that make up all cells. The relationships between different terrestrial
life forms are quantitatively explicit in the now-emerging maps of the
course of evolution, phylogenetic trees based on DNA sequences. Even if
potentially alien organisms were to present the same biochemistry as
seen in terrestrial organisms, genetic sequences could provide criteria
to distinguish them if they are of different evolutionary sources.

The gene sequence-based overview of terrestrial biological diversity is
embodied in phylogenetic trees, relatedness diagrams such as that shown
in Fig. 1 (9). The construction of a
phylogenetic tree is conceptually simple. The number of differences
between pairs of corresponding sequences from different organisms is
taken to be some measure of the “evolutionary distance” that
separates them. Pair-wise differences between the sequences of many
organisms can be used to infer maps of the evolutionary paths that led
to the modern-day sequences. The phylogenetic tree shown in Fig. 1 is
based on small-subunit ribosomal RNA (rRNA) gene sequences, but the
same topology results from comparing sequences of any other genes
involved in the nucleic acid-based information-processing system of
cells, the core of genetic continuity. On the other hand, phylogenetic
trees based on metabolic genes, those involved in manipulation of small
molecules and in interaction with the environment, sometimes are not
congruent with the rRNA topology. Such genes do not offer any
consistent alternative to the rRNA tree, however. Consequently,
patterns that deviate from the rRNA tree probably are best interpreted
to reflect lateral transfers of genes or even the intermixing of
genomes in the course of evolution (10). The genome of any particular
organism is comprised of genes derived evolutionarily from both
vertical and lateral transmission.

Universal phylogenetic tree based on small-subunit rRNA sequences.
Sixty-four rRNA sequences representative of all known phylogenetic
domains were aligned, and a tree was constructed by using the program
fastdnaml, correcting for multiple and back mutations. That
tree was modified to the composite one shown by trimming lineages and
adjusting branch points to incorporate results of other analyses.
Evolutionary distance (sequence difference) between the species shown
is read along line segments. The scale bar corresponds to 0.1 changes
per nucleotide. [Reproduced with permission from ref. 9 (Copyright
1997, American Association for the Advancement of Science).]

Phylogenetic trees are rough maps of the evolution and
diversification of life on Earth. From the standpoint of both sequence
divergence and complexity, most of Earth's life is seen to be
microbial in nature, which is surely what we need to expect of life
that might occur elsewhere in our solar system. Some of the conclusions
that can be drawn from the molecular trees verify previously advanced
biological hypotheses. For instance, the molecular trees confirm what
was once a hypothesis, that the major organelles, mitochondria and
chloroplasts, were derived from bacteria, proteobacteria and
cyanobacteria, respectively. The biochemical trait of
oxygenic photosynthesis arose with the cyanobacterial radiation
(indicated in Fig. 1 by the lines leading to Synechococcus,
Gloeobacter, and chloroplast). Because the cyanobacterial
radiation is peripheral in the tree, early life must have been
anaerobic and most of bacterial diversification must have happened
before the availability of oxygen.

Other conclusions from the molecular trees clarify relationships
among terrestrial life. The molecular trees show that Earth's main
lines of descent fall into three relatedness groups, the
“domains” of Archaea, Bacteria, and Eucarya (eukaryotes). The
point of origin of the lines leading to the modern domains cannot be
determined by using rRNA gene sequences alone. Comparison of gene
family sequences such as the protein synthesis factors Ef-Tu and Ef-G,
that diverged before the last common ancestor of life, however,
indicate an origin deep on the bacterial line as shown in Fig. 1. This
relationship means that Archaea and Eucarya shared common ancestry
subsequent to the separation of their common ancestor from Bacteria.
Biochemical properties of the organisms are consistent with this
conclusion. For example, the transcription and translation machineries
of modern-day representatives of Archaea and Eucarya are far more
similar to one another than either is to corresponding functions in
Bacteria. This result shows that the eucaryal nuclear line of descent
is not a relatively recent derivative of symbiosis, rather, is as old
as the line of Archaea. This result also indicates that the common
textbook presentation of life as divided into two categories,
prokaryote and eukaryote, is incomplete. Rather, terrestrial life is of
three kinds: archaeal, bacterial, and eucaryal, distinct from one
another in fundamental ways.

Gene sequences that are common to all organisms are incisive signatures
of terrestrial origin. This is because organisms with independent
origins are unlikely to have evolved identical genetic sequences, even
if the chemical structures of the subunits that comprise the genetic
information were identical. Thus, in gene sequences we can recognize
terrestrial life, distinguish it from life derived from a different
evolutionary origin even in the face of substantial biochemical
similarity. This would become a significant issue if life—or its
remains—were discovered on another body in the solar system.

Because planetary systems are formed by accretion, I think it
unlikely that life on another body in the solar system arose
independently of terrestrial life. It is now clear from meteorite
studies that bodies can be transported from one planet to another, for
instance from Mars to Earth, without excessive heating that would
sterilize microbial organisms (11). Although such transfer events are
now rare, they must have been far more frequent during the accretion of
the planets. Large-scale infall, blasting ejecta throughout the forming
solar system, probably extended until at least about 4 billion years
ago and so probably overlapped with the processes that resulted in the
origin of life. In principle, life, regardless of where it arose, could
have survived interplanetary transport and seeded the solar system
wherever conditions occur that are permissible to life. So, if we go to
Mars or Europa and find living creatures there, and read their rRNA
genes, we should not be surprised if the sequences fall into our own
relatedness group, as articulated in the tree of life.

Acknowledgments

My research activities are supported by the National Institutes of
Health, National Science Foundation, and the National Aeronautics and
Space Administration Astrobiology Institute.

Bacteria could help tackle the growing mountains of e-waste that plague the planet. Although researchers are a long way from optimizing the approach, some are already confident enough to pursue commercial ventures.

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